Demi Lovato is one of the best young singers on the market right now. She had a really unique situation in that she sings very different kinds of music from the music she listens to. The music business is filled with people who want to change you. How do you do that but stay true to yourself?
LEARN TO REMIX. Demi Lovato likes metal and hardcore music, but she doesn't necessarily sing that. Why not? Well, the first thing is to realize that no matter what type of music it is, singers sing about the same things. We're all people. We all want to be in love, we all hurt after breakups, we all go to parties, we all get depressed, we all dance. ALL singers sing about this. You know what a remix is, right? So if you want to sing, you have to learn how to remix stuff. That's what a music producer does. If you have a really angry song idea, but the beat is happy, you might have to switch your rhythm, or use irony (English class!) or sing a certain note instead of scream it.
EXERCISE. A good voice requires a good body. You have to exercise and be in shape. Your vocal chords are muscles just like your arms or legs, and they must be worked out. So practice singing out, and even when you're speaking in class, speak out. Start from your gut and use your lips to shape the words. That's how Demi Lovato sings.
CONSTANT PRACTICE. You can Google or Youtube practice techniques, because you will be practicing a lot if you want to be a singer. Many young singers think they want to be singers because of the red carpets, the Hollywood celebrities, and riding in limos. What they don't realize is that before all that, you will be by yourself, trying to think up hit songs and trying to get your stage show perfect. It's hard work and takes sacrifice. Many young singers don't even get to go their proms, so just realize what you're giving up and don't take it lightly. Practice, practice, practice.